TCO – Tetrazine Ligation(Figure 1). Schematic representation of a TCO–Tz ligation reaction.At low biomolecule concentrations (e.g. < 5 μM) where SPAAC falls short due to modest kinetics (e.g. 0.3 to 2.3 M–1s–1), and where copper–catalyzed alkyne–azide cycloaddition click reaction might compromise system viability, trans–cyclooctene–tetrazine (TCO–Tz) is the reaction pair of choice. This ligation chemistry is based on an inverse–dema…